Tractors and other vehicles often have systems or mechanisms which prevent activation of the engine starter unless the transmission is disengaged. Such vehicles have required that the transmission and perhaps other drives be in a disengaged condition, either by placing a control device in a neutral state or by disengaging a clutch, before electrical power can be supplied to the starter. This prevents sudden movement of the vehicle when the engine starts, potentially causing injury to people or damage to property.
In electronic transmission control systems, a small snap action switch has been used to prevent starter engagement unless the transmission control lever is in an appropriate position. This switch is not particularly reliable and is often difficult to design into compact transmission control devices. An alternative would be to have an electronic control unit read the transmission control sensing means and have it determine if starting is acceptable, but this requires that the control unit be powered during the start cycle. Such a requirement necessitates a more capable power supply for the microcontroller, adding expense and volume to the unit. Additionally, practical application of the snap action switch results in appreciable current being passed through the wires and connectors leading to the switch.